Brushbuffalo, could you explain what geologic events would cause this giant chasm on Vesper Peak?

Blue Mountain views 12/23/17

And similar ones on Big Four?

Blue Mountain views 12/23/17

Nancyann, when you see an arrow-straight feature of size, it is almost always either a fault, a joint (fracture), or a man-made feature. There is a significant fault (Darrington- Devils Mountain fault zone) extending near the summit of Vesper Peak and there are contacts between three rock formations of contrasting resistance to erosion: granitic rock ( tonalite), metavolcanic rock, and sandstone. Faults and large joints extending up/down slope often channel surface runoff, and the prominent gash is most likely formed by that process....water running along a zone of weakness (fault, joint, or contact), thus deepening it.

Big Four is composed mainly of metavolcanic rocks. The very steep but not particularly deep valleys on the south aspect of Big Four are due to stream flow but not along any particular zone of weakness. Note how they are rather evenly spaced with ridges intervening.
You can confidently distinguish between stream-eroded valleys (V-shape cross sectional profile) from glacial valleys (U-shaped).

For those who don't know Vesper, this south aspect is readily visible from many areas around the Seattle area. And the north side offers some of the finest granite rock for climbing in the state. I am enjoying these interesting conversations as well.

Brushbuffalo, could you discuss a little about the geology of adjacent Sperry Peak? The climbing rock on parts of Sperry is very hard and has almost a quartzite like nature, with embedded round pebbles.

Brushbuffalo, could you discuss a little about the geology of adjacent Sperry Peak? The climbing rock on parts of Sperry is very hard and has almost a quartzite like nature, with embedded round pebbles.

Sure, gb. Thanks for the question. Although I have been up Vesper I haven't ventured over to Sperry yet. So let's turn to the authority! Geologic Map of the Sauk River 30- by 60- Minute Quadrangle, Washington, by Tabor et al ,
Sperry Peak, just northeast of Vesper, is composed of sedimentary rock, mapped as formation Tbs. Most of this formation is sandstone but Tbs also contains conglomerate. Come to think of it, I remember seeing conglomerate boulders poking above snowpack on the trail to Vesper that must have been shed from Sperry.

Here is a detail to look for on your next Sperry trip. Quoting from the map's explanatory notes: "Within the Darrington-Devils Mountain Fault Zone, conglomerate clasts are highly stretched, roughly horizontal, and parallel to the faults, suggesting strike-slip movement along the fault zone."

[An active modern-day strike-slip fault ( horizontal movement along the fault trace) is the famous San Andreas Fault Zone in California.]

NorthBen, thanks for the picture of that very interesting boulder. I have some comments based on location of Gardner Mountain and using the Geologic Map of the North Cascade Range by Ralph Haugerud and Rowland Tabor. To give you a more definite answer I would return to that fun choss pile again and carefully observe the bedrock outcrops over a wide area. I camped on top of Gardner many years ago, but was more interested in the goats and the rare "green flash" I saw at sunset so didn't pay enough attention to the rock.

Part of the boulder is a course-grained conglomerate fragment consisting of subangular to subrounded clasts, including at least one granitic clast. The boulder is likely a bit of the bedding contact between a conglomerate and a very well-sorted sandstone. There are other thoughts I have about this rock that involve sketches, arm-waving, and other things not possible in this reply to your question. Fascinating, but I really need to be there to look around! Sometimes it's hard to identify a single "tree" outside of the context of the " forest."

There are several different formations that occur on and around Gardner Mountain, which together are known as "rocks in the Methow block." You may have an example of the formation abbreviated Kpv, and this is part of the Pasayten Group, which also has a companion formation Kps ( sedimentary) found along the lower western base of Gardner over toward Abernathy Peak.

Kpv is "predominantly andesitic breccia and tuff; locally fluviatile [river] maroon siltstone, sandstone, and conglomerate. About 90 million years ago, volcanoes erupted on the floodplains of rivers that flowed over sediments of the former Methow Ocean, burying both the river deposits ( Kps) and the underlying Methow Ocean sediments under volcanic rocks." However, the individual particles (clasts) in your boulder don't look to be volcanic.
The maroon color, which is much more distinctive in some locations in the Methow region, is due to oxidation of stream-deposited sediment.

There is another formation outcropping near the top of Gardner, Ktf. I recall seeing spectacular conglomerate boulders of Ktf on the PCT on the east slope of Powder Mountain between Rock Pass and Woody Pass, but I digress. Ktf is the Three Fools sequence, which is " predominantly thick-bedded sandstone, minor thin-bedded sandstone, argillite, and conglomerate deposited in deep-water submarine fans. Between 105 and 110 million years ago, ......volcanic islands formed in the western part of the Methow Ocean. The weight of these islands caused the ocean to deepen [isostatic adjustment]. Occasional flows of sediment-rich water ( turbidity currents), perhaps created by submarine landslides or storm waves, cascaded from the east and west into the depths to create these turbidite deposits." Problem with this is that the boulder is quite different than a turbidite.

Please pardon my vagueness in this response. "it could be this, or that, or something else".....bottom line is I'm not very confident in a definitive ID as far as which formation this boulder was broken off from.. . I often am challenged trying to identify a loose rock from a single picture without... or even with....scale, and without seeing it in field relationships. But I enjoy the challenge. It's fun to be conjectural, and probably makes for amusing (and maybe even occasionally informative) reading. The North Cascades have some of the most complex geology anywhere. It has taken field geologists many decades to try to figure it out.....and they are still trying!

Friends,
This thread is called "Name That Rock" [changed to "What's this rock? What's that landform?"] and has attracted a moderate amount of attention.
In reading miscellaneous trip reports, I frequently see pictures of grand landscapes with fascinating landforms and also images of rocks and minerals. Sometimes the OP will include a question about what they are seeing. I feel uncomfortable about just barging in with an opinion without being asked. To me, that is rude. To those to whom I have done that in the past, please accept my apology.

Right now is slack season for many who otherwise might be traipsing about on exposed rocks and landforms. if you have questions about geologic things you have seen in your past travels, add a post to this thread and ask a question. If you see something geologic in another's post, ask me also, but out of courtesy please sent that person a pm asking for their permission to have me look at the image they posted.

Please be aware of hints to allow me to make a reasonable explanation ( see the initial post): location, as precise as possible, and scale (size) if possible. Although I have been fortunate to travel, I do better with PNW stuff because I am more familiar with this region, plus I have geologic maps that help. Fossil images mess with my head unless they are common and obvious ( I'm not a paleontologist).🤔

No promises for positive identification, but it is fun to offer even a "best educated guess". With close to half a century of college geology teaching as well as extensive field work, I have seen a lot of rocks, structures, and landforms.
We can all learn together!
Doug

I feel uncomfortable about just barging in with an opinion without being asked. It is rude. To those to whom I have done that in the past, please accept my apology.

I, for one, would welcome unsolicited commentary on geology appearing in my pictures! If I've posted a trip report here, I assume it's an invitation for others to comment on it. No rudeness at all.

But solicited opinions are great too! Here's one for which my assumption was adamantly refuted by a geologist friend. Her explanation makes no sense to me, so I'll ask for a second opinion. (I won't mention either my assumption or her opinion so as not to taint the well.)

What geological process caused the paper-smooth side of this rock? This is right on the PCT in the Goat Rocks, and is about the size of a chest freezer, give or take. Many of us here have probably seen it.

Hi texasbb! Thanks for the post.
I have a couple of possibilities for that interesting rock. The material itself is almost certainly andesite of the extinct "Goat Rocks" volcano. The splintered fracture pattern in not unusual in andesite. Joints are a major reason that rock in volcanoes in the Cascades is notoriously chossy.

I think the prominent flattened face on the right of the block in the images that is roughly perpendicular to the two sets of closely-spaced joints ( fractures) forming the splinters is another joint formed after the other two joint sets. The rusty color is due to chemical weathering across the exposed joint surface, and such weathering also is evident on surfaces of several exposed splinters in the lower left of the first picture.

Another possibility is that the flattened surface is a glacially planed surface and the block was ripped out of and rotated from its bedrock location. Although glacially smoothed and polished rock surfaces are very common in the Cascades, overriding movement by ice on this thoroughly fractured rock would most likely leave a rougher surface due to splintered pieces being ripped out one by one to a much greater extent.

Personally I favor the first explanation (jointing). A visit to the site to look around would easily resolve which of these ( or other) explanations is correct.

I have been there or very near there, but it was in about 1964, so pardon my fuzzy memory.

Personally I favor the first explanation (jointing). A visit to the site to look around would easily resolve which of these ( or other) explanations is correct.

Well, that's two geologists against one know-nothing (well, two know-nothings if mike above accepts that label ). My friend also said a joint, including mention of the chemical weathering.

If I understand the (backwards IMHO) nomenclature, a joint is a fracture, right? One resulting from tension--i.e., the rock was pulled apart? I don't see how that could have happened without shredding or separating all those loose mini-slabs. I can, however, imagine it getting sanded flat by glacial movement without getting torn apart because there would be so much pressure holding it together from all that ice on top of and around it. Guess I better stick to other subjects!

I don't know if seeing a map would be helpful to you, but the rock is right here (the blue polygon).

Yes, a joint is a fracture, just a crack. Joints are similar to faults except the latter involves differential movement on each side of the fault plane relative to the other.
Joints can be caused by tension but also by compression or shear. They frequently occur in groups with more or less parallel alignment in what are called joint sets due to a specific application and orientation of stress.

As mentioned, it would be a quite simple matter to figure out the cause by examining the surrounding geologic and geomorphic setting. However, without seeing the setting, I favor jointing...although without question that area was extensively glaciated during the Pleistocene.

Thanks for the picture. Inconclusive, however. If I was there I would be walking and looking over a wide area surrounding the boulder. I would examine lots of other boulders and especially look for outcrops of bedrock. Otherwise it's similar to finding the odd ponderosa pine in a Douglas fir forest.

You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot vote in polls in this forumYou cannot attach files in this forumYou can download files in this forum